What is an Interferometer?

An interferometer is a precision measurement device that works based on interference, a phenomenon where two or more waves superimpose to produce an interference pattern in which the initial light intensities are redistributed and the resultant wave has higher, equal, or lower amplitudes. They are used in various industrial and academic applications like sensing, distance measurement, tracking & navigation, spectral analysis, and for surface quality analysis of various components and systems like lasers, optics in CD & DVD drivers, machine parts, etc.

Interferometers usually split a beam of light into two using a beamsplitter, a semitransparent mirror, and allow them to propagate through different paths as shown in the above figure. One of them is taken as the reference beam while the other is used for sampling purposes and is called the sensing beam. The sensing beam either is illuminated on a sample and gets reflected or passes through the sample. This reflected or transmitted beam will be modified from the incident sensing beam by the information from the sample. Superimposing it on the reference beam will result in an interference pattern that has all the necessary information about the sample. Hence, the required details of the sample under observation can be obtained by analyzing the pattern. This pattern is projected onto a screen, imaging detector, or camera.

The interference pattern formed is due to the superposition or overlap of both beams and it will have bright and dark areas or bands known as interference fringes. The bright fringes are regions of constructive interference and the dark fringes are regions of destructive interference.

Constructive interference takes place when the waves superimpose in phase, the crests and valleys of both waves match with each other, at that location resulting in a higher amplitude wave. Destructive interference occurs when the two superimposing waves are out-of-phase, i.e., the crest of one wave coincides with the valley of the other, resulting in a lower or zero amplitude wave. If the superposition of the two waves takes place with a phase difference that is in between, then it will result in an intensity depending on the degree of phase difference. These phase differences arise due to the interaction of the sample with the sensing beam.

If two conventionally used light sources, for example, lightbulbs, are placed close to each other, no interference patterns or fringes can be observed. Even though the emitted light waves superimposing at each location on a screen or a wall have certain phase differences from each other, the randomness in these instantaneous phase differences will make it difficult for human eyes or other detectors to perceive them or to adapt to rapid amplitude changes. Hence, an average illumination is being perceived. So, maintaining a constant phase difference between the sources is very important for observing or detecting interference patterns. This property is known as coherence. When the waves emitted by two sources have the same frequency and a constant phase difference, the sources are said to be coherent. Otherwise, they are known as incoherent sources.

Interference patterns can be observed on soap bubbles, oil films floating on water, etc. and the phenomenon of interference can be proved using Young’s double slit experiment where light waves from two secondary light sources (S2) derived from a point light source (S1) using small apertures produces interference pattern with bright and dark fringes on a screen (F) placed at a distance from them as shown in the above figure.

Modern interferometers use image sensors to capture the interference pattern and store them as interferograms. They can be easily analyzed using sophisticated software packages to extract necessary information and to re-create original images from them. 

There are different types of interferometers such as Michelson, Fabry-Perot, Fizeau, Mach-Zehnder, Sagnac, Twyman-Green interferometers, etc.

Interferometers in Astronomy

In astronomy, interferometry can be used to obtain high-resolution information about stars or other heavenly bodies. Signals from an assembly or array of multiple small telescopes or mirrors can be used to achieve a resolution equivalent to a large telescope that has the dimension of this assembly. A complex mirror system supports this assembly to bring out the light signals from each telescope and is superimposed to produce interference fringes from which high-quality, finely detailed images can be re-created or required information be obtained.

These interferometers are known as stellar interferometers and they use the property of spatial coherence. The spatial coherence of transversely separated sources decreases as the angle subtended by it increases. So, the angle subtended by the star on the earth can be determined by analyzing the spatial coherence in the interference fringes generated by the signals from two or more transversely separated telescopes. The resolution in angle obtained from spatial coherence increases as the separation of these telescopes increases. Combining details from other sources with the result of stellar interferometry, diameter, distance, surface intensity distribution, etc. can be deduced for the given star.

Interferometers in the Medical field

Interferometers can also be incorporated into optical fiber systems that are used in medical instruments like endoscopes, needles, and catheters which allow in vivo study of biological cells, tissues, and internal organs. They can perform in vivo medical imaging, diagnosis, monitoring, and minimally invasive surgeries. They use the advancements in the knowledge of the interaction between light and biomaterials.

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